Programming iOS 6, 3rd Edition

Debugging

Debugging is the art of figuring out what’s wrong with the behavior of your app as it runs. I divide this art into two main techniques: caveman debugging and pausing your running app.

Caveman Debugging

Caveman debugging consists of altering your code, usually temporarily, typically by adding code to dump informative messages into the console.

The standard command for sending a message to the console is NSLog. It’s a C function, and it takes an NSString which operates as a format string, followed by the format arguments.

A format string is a string (here, an NSString) containing symbols called format specifiers, for which values (the format arguments) will be substituted at runtime. All format specifiers begin with a percent sign (%), so the only way to enter a literal percent sign in a format string is as a double percent sign (%%). The character(s) following the percent sign specify the type of value that will be supplied at runtime. The most common format specifiers are %@ (an object reference), %i (an integer), %f (a float), and %p (a pointer, usually an object reference, shown as the address in memory pointed to, useful for making certain that two references refer to the same instance). For example:

NSLog(@"the window: %@", self.window);

In that example, self.window is the first (and only) format argument, so its value will be substituted for the first (and only) format specifier, %@, when the format string is printed in the console. Thus the console output looks something ...